Three US troops were among four killed in an aircraft crash in Afghanistan, the military said on Friday, in a fresh setback to foreign forces in the increasingly deadly fight with the Taliban. The US Air Force CV-22 Osprey, which takes off like a helicopter but flies like a plane, came down in Zabul province in the restive south of the country, where most of the fighting in the eight-year insurgency has been concentrated. "The cause of the crash is unknown at this time," NATO's International Security Assistance Force (ISAF) said in a statement after the Taliban claimed that they shot down the distinctive tilt-rotor aircraft near Qalat city. A civilian employee also died in Thursday night's crash -- the third in recent weeks involving a NATO helicopter -- while an unspecified number of service personnel were injured. Mohammad Jan Rasool Yar, spokesman for the governor of Zabul, said a technical fault caused the crash.